A Study on Heat Cured Fly Ash based Geo Polymer Concrete

Abstract:- Continuous increase in production of cement causes large amount of carbon dioxide emission which results in green house effect. In order to overcome this problem many researchers have put in their efforts to achieve optimum strength of concrete by replacing cement with fly ash, when it combine with alkaline solution to produce Geopolymers concrete(GPC). GPC is an improved way of concreting execution made by complete elimination of ordinary Portland cement. GPC were synthesized from low calcium fly ash, activated by combination of Sodium Hydroxide and Sodium Silicate solution. This report is an attempt to find out suitable utilization of fly ash by studying the compressive strength of GPC and to observe durability characteristics of GPC. In this experimental study different concentrations of alkaline liquid are being used. Mix samples of different molarities were prepared to study the influence of alkaline solution on compressive strength of GPC. Increased alkaline solution concentration proved to have positive effect on Geopolymerization process and this is revealed by the improved compressive strength.

INTRODUCTION

The global demand of cement for construction of infrastructures is continuously increasing in order to maintain the ongoing growth and accommodate the needs of the increasing population. OPC has been traditionally used as the binder in concrete. About 1 tonne of carbon dioxide is emitted into the atmosphere in the production process of 1 tonne of cement. This makes a significant contribution to the global greenhouse gas emission. Therefore,

development of alternative binders utilising industrial by- products is necessary to reduce the carbon footprint of the construction industry. Geo polymer is an emerging alternative binder for concrete that uses by-product materials. A base material that is rich in Silicon (Si) and Aluminium (Al) is reacted by an alkaline solution to produce the geopolymer binder. Source materials such as fly ash, metakaolin and blast furnace slag can be used to make geo polymer.

Fly ash blended with blast furnace slag and rice husk ash has also been used as the base material for geopolymer. The product of the reaction is an inorganic polymer which binds the aggregates together to make geopolymer concrete. The coal- fired power stations worldwide generate substantial amount of fly ash as a by- product that can be efficiently used in geopolymer concrete to help reduce the carbon footprint of concrete production.

The results of recent studies have shown the potential use of heat-cured fly ash based geopolymer concrete as a construction material. As a relatively new material, it is necessary to study the various properties of GPC as compared to the traditional OPC concrete in order to determine its suitability for structural applications. The ongoing research on fly ash-based geopolymer concrete studied several short-term and long-term properties. It was shown that heat-cured geopolymer concrete possesses the properties of high compressive strength, low drying shrinkage and creep, and good

resistance to sulphate and acid. Geopolymer concrete was found to have higher bond strength with reinforcing steel and relatively higher splitting tensile strength than OPC concrete. Geopolymer concrete beams and columns were tested to failure and they showed similar or better performance as compared to OPC concrete members. Heat-cured geopolymer concrete showed higher residual strength than OPC concrete cylinders after exposure to high temperature heat of up to 8000 C

.Therefore heat-cured geopolymer concrete is

considered as an ideal material for precast concrete structural members.

Development of the constitutive model for a material requires its fracture parameters. The fracture characteristics of a material are used to describe the formation and propagation of cracks in the material. The crack path through a composite material such as concrete is dependent on the mechanical interaction between the aggregates and the binder matrix. Fracture energy of a composite material depends on the deviation of the crack path from an idealized crack plane. Since the binder in geopolymer concrete is different from that in OPC concrete, the effect of the interaction between the aggregates and the geopolymer binder needs to be investigated. Thus, it is necessary to study the fracture parameters of geopolymer concrete to understand its failure behaviour. In this study, the fracture properties of heat cured fly ash based geopolymer concrete specimens were determined from three-point bending test of notched beams. Fracture energy and the critical stress intensity factor were also determined for OPC concrete specimens to compare with those of geopolymer

concrete specimens of similar compressive strengths and containing the same aggregates. The fracture behaviours of both types of concrete were compared using the test results.

fly ash was 50Âµ m. Sodium Silicate solution (Na2SiO3) and Sodium Hydroxide (NaOH) were used as Alkali activators. Coarse aggregate with a maximum size of 20mm diameter with a specific gravity of 2.85 was used for making Geopolymer concrete. Sand used for the experiment was passed through the 4.75mm IS Sieve with a fineness modulus of 2.46.

Mix proportion, mixing and casting.

Mix chosen in this study is 1:1.5:3 with 100% replacement of cement with fly ash. Alkaline liquid was a combination of Sodium hydroxide and Sodium Silicate solution. Sodium hydroxide and Sodium Silicate obtained as pellets were dissolved in distilled water to form the alkaline liquid. In order to study the effects of alkaline solution on the geopolymer concrete properties, three concentrations of alkaline solutions 8, 10, 12 molar were used. Mix proportions are shown in Table 1.

Table 1 Mix proportion of geo polymer concrete

Mix Proportion(kg/m )

Mix No

Fly ash

FA

CA

Alkaline

liquid

Liquid

Water

S1

418

628

1256

10 M

150

50

S2

418

628

1256

10 M

100

100

S3

418

628

1256

8 M

150

50

S4

418

628

1256

8 M

100

100

S5

418

628

1256

8 M

50

150

S6

418

628

1256

12 M

150

50

S7

418

628

1256

12 M

100

100

S8

418

628

1256

12 M

150

150

The coarse aggregate and sand is saturated surface dry condition was first mixed in laboratory pan mixer with the fly ash for about three minutes. At the end of this mixing, the alkaline solutions and extra water were added to the dry materials and the mixing continued for another four minutes. Immediately after mixing, the fresh concrete was cast into moulds. All cubes were cast in two layers. Each layer was compacted into limited capacity of the laboratory mixer. The slump of every batch of fresh concrete was measured in order to observe the consistency of the mixtures. Casted cubes were kept in oven for 48 hours at the temperature of

700C for curing. After curing, the cubes were

removed from the chamber and left air-dry at room temperature for another 24 hours before demoulding.The test specimens were then left in the laboratory ambient conditions until the day of testing. The laboratory temperature varied between

250C and 350C during that period.

Testing Detail

Void Content

The void content of Geopolymer concrete was tested using the casted cubes. The void content was determined in accordance with ASTM and calculated using Eq (1).The reported void contents were the average of three samples

Table 2 Total void ratio

VT = (T-D)*100/ T———-(1)

T = Ms /Vs

Mix

Void content%

S1

15.5

S2

16.6

S3

17.1

S4

17.7

S5

18.6

S6

10.5

S7

11.1

S8

12.3

Where VT is the void content (%),

T is the theoretical density of Geopolymer concrete computed on an air free basis (kg/m3).

Ms is the total mass of all the materials batched (kg), VS is the sum of absolute volumes of component ingredients in the batch (m3).

Compressive strength

The compressive strength was tested at the age of 3rd day. The crushing strength of concrete cube is

determined by applying a compressive load at the rate of 2.88N/mm2, till the specimen fails.

Void Ratio Vs Alkaline Concentration

20

12M

Void Ratio in %

Void Ratio in %

15 Concentra

tion

after 28 days. It has been found that age does not have a significant effect on strength of Geopolymers after completion of heating curing cycle.

Strength of Geopolymer concrete

10 10M

Concentra

5 tion

8M

0 Concentra

25% 50% 75% tion

28.8

11.6

Compressive Strength

32.6 30.7 30.2

24.2

12.25 9.7

RESULTS AND DISCUSSION

3.1 Void Content

The results of void content are summarized in Table 2.The void content of geopolymer concrete were relatively low between 10.5% and 18.6%. Generally the void content of Portland cement concrete depends on the gradation of aggregate and the method of compaction. However in this test, the gradation of aggregate and the method of compaction were not varied. The results however indicated that voids content in this study slightly decreased with the increase in the alkaline concentration.

For instance the void content of S6, S7, S8 were

10.5%, 10.8% and 11.2% respectively. The adding of alkali liquid in the mixture increased the paste content and the excess paste fill the voids resulting in a dense concrete with low void content.

Compressive Strength

Compressive strength tests of all specimens were conducted using a compressive testing machine. A minimum of three specimens (150mm*150mm ) cubes for each type were tested for 3-day compressive strengths after casting, which is equivalent to a typical OPC strength development

The strength of the fly ash based geopolymer concrete is significantly increased the geopolymer paste undergoes high early strength development at an accelerated rate. This behaviour is the characteristics of the quick geopolymerization process, which contrasts with the hydration process of OPC that gains strength over longer time periods. In geopolymers, alumino-silicate gel is the major binding phase that provides interparticle bonding, which in turn enhances the macroscopic strength.

Compressive strength of Geopolymer concrete increases marginally with an increase in alkaline content. For example compressive strength of 9.70

24.2, 11.6 28.8, 30.2 32.6 MPa were obtained for GPC with 8 M, 10M and 12M respectively. With regard to Sodium Hydroxide concentration, the optimum concentration to produce GPC is 12M. Increasing the concentration to 12M increases the compressive strength.

Relationship of void content and compressive strength

The relationship between void content and compressive strength can be shown using the exponential curve as in figure. It can be seen that compressive strength increases as void content decreases.

CONCLUSION

In order to expand the use of fly ash Geopolymer concrete which can be prepared easily from alkali activated fly ash and coarse aggregate. The void content and compressive strength were determined. Compressive strength between 9.7 and

were obtained according to the molarity of the alkaline content. These values show that optimum alkaline content can be chosen between 10M and

12M according to the requirement. In addition the relationship of void content alkaline content, compressive strength alkaline content of GPC was shown in graph. It has therefore been demonstrated that fly ash geo polymer concrete could be used as replacement for ordinary Portland cement concrete with acceptable strength.